The simplest interferometer is a Michelson plane mirror interferometer presented schematically in FIG. 1. The main components of this interferometer are the beamsplitter, the fixed mirror and the movable mirror. A light beam hits the beamsplitter, whereupon part of it passes through and is reflected from the fixed mirror back to the beamsplitter and therefrom to the receiver, which may be e.g. a photocell or a human eye. Part of the light beam is reflected from the beamsplitter onto the movable mirror from which it is reflected back to the beamsplitter and further to the receiver. The beams incident on the receiver from the fixed mirror and the movable mirror interfere. If the distances of both mirrors to the beamsplitter are exactly equal, said distances include the same number of wavelengths of the used light. If the movable mirror is moved closer to or farther away from the beamsplitter in the way indicated by the arrow, the receiver can register interference maxima at a distance of half of the wavelength.
Interferometers are used e.g. for the measurement of distances with very high accuracy, for the mapping of unevenness of various surfaces, and for the determination of wavelength or wavelengths (spectra) of electromagnetic radiation.
Spectrometry is the widest application area of interferometers. In this application, it is important that the moving mirror is capable of being moved with high accuracy without tilting the mirror. The maximum allowable tilt angle must obey the equation .beta.&lt;.lambda./8D where D is the diameter of the mirror and .lambda. is the wavelength. There have been attempts to solve the problem e.g. by employing cube corner mirrors as the fixed mirror and the moving mirror. Another possibility is to employ a so-called dynamic alignment system wherein either the moving or the fixed mirror is continuously adjusted to maintain the adjustments of the interferometer. However, a new problem arises from the application of cube corner mirrors i.e. their lateral movement. Furthermore, cube corner mirrors are expensive and they have insufficient accuracy especially in the UV range. The dynamic alignment system is very sensitive to disturbances caused by mechanical vibrations. Furthermore, the linear path is mechanically very sensitive to external disturbances which always contain components in the direction of the motion.
The Perkin Elmer Dynascan.TM. 2000 instrument, the structure of which is shown in FIG. 2, represents an improvement on the classical Michelson interferometer. The beamsplitter and the mirrors PP1 and PP2 reflecting the light beams back are placed immovable with respect to each other. The optical path difference of the beams from the beamsplitter is accomplished by means of two pairs of mirrors HP and HP' so that said pairs of mirrors are placed on a rigid mount which is rotatable around an axis marked with a + in the manner indicated by an arrow. The advantage with this so-called swinging interferometer is that the mirrors themselves need not be moved linearly with respect to the mount. The problem with this instrument is, however, the complicated structure and especially its large size which limits its use in spectrometers.
The U.S. Pat. No. 4,915,502 presents an improved swinging interferometer whose design is shown in FIG. 3. This interferometer differs, in principle, from the Perkin Elmer Dynascan.TM. 2000 instrument described above in that both beams S1 and S2 from the beamsplitter pass through the one and the same pair of mirrors HP. Thus the size of the instrument becomes smaller than in said Perkin Elmer solution. In this solution, the optical path difference can be lengthened by lengthening the "corridor" of the pair of mirrors thereby allowing both beams to pass several times between the pair of mirrors HP.
Though the solution described in the U.S. Pat. No. 4,915,502 represents an improvement on the prior art swinging interferometers, this instrument has obvious disadvantages. The main reason for the inaccuracy of an interferometer is the fact that due to pressure and temperature variations the mount deforms so that its one edge stretches or compresses more than the other or that the mount is subjected to torsion forces whereupon opposite sides of the mount twist in opposite directions. In the solution of said US patent the beamsplitter and the retroreflecting mirror assembly (consisting of two separate plane mirrors PP1 and PP2) which are both supported on the mount, are situated apart from each other. For this reason, even minute forces acting on the mount cause considerable measurement perturbations.